Here is an area that I continue to struggle with and one that I've have improved upon with help from Brian Tracy who writes the following:
The very worst use of your time is to do what need not be done at all. The Pareto Principle says that 20% of your activities will account for 80% of the value of your activities. This means that, if you have a list of ten items to accomplish, two of those items will be worth more than the other eight items altogether.
To achieve great things, you must always be concentrating on the small number of activities that contribute the greatest value to your life and your work.
Determine the Consequences
The value of anything in your order of priorities can be measured by assessing the potential consequences of doing it or not doing it. Something that is important has significant consequences to your life and your career. Something that is unimportant has few or no consequences of significance to your life or career. The mark of the superior thinker is your ability to consider possible consequences before you begin.
Ask the Key Question
Continually ask yourself, "What is the most valuable use of my time, right now?" And whatever it is, work on that. Your ability to discipline yourself to work on those few tasks that can make the greatest difference in your life is the key quality that makes everything else possible for you.
Action Exercises
Here is how you can apply this law immediately:
First, make a list of everything that you do as a part of your job. Now, analyze the list and select the three to five things that are more important than everything else put together.
Second, imagine that you are going to receive a $100,000 bonus at the end of the month if you can work on your highest priority items every minute of the day. How would that change your behavior? What would you do differently?